Since the creation of the first integrated circuit in 1960, there has been an ever increasing density of devices manufacturable on semiconductor substrates. Silicon technology has remained the dominant force in integrated circuit fabrication. Very large scale integration, or VLSI, devices having more than 100,000 devices per chip, have become foundational to computer technologies and many related technologies. The increasing device count has been accompanied by a shrinking (minimum) feature size, now diminished to less than 1 .mu.m.
As the size of features shrink below 1 .mu.m, and chip sizes increase beyond 1.0 cm.sup.2, polycide sheet resistances of 1-5 ohms/sq become the limiting performance factor for VLSI circuits. In these cases it is necessary to use even lower resistance interconnects, possibly made of metal films (P. Burggraaf, "Silicide Technology Spotlight", Semi International., May '85, p. 293).
Several nonferrous and refractory metals such as Al, Ti, W, Mo, and Ta have been considered by the electronics industry as interconnect materials. The application of these metal interconnects has been extensively studied by researchers in the electronics industry. The selection and use of these materials in VLSI devices depends greatly upon their physical and chemical properties. For example, aluminum is widely used by the electronics industry for VLSI interconnects because of its high conductivity. However, aluminum suffers from an inability to withstand high temperature processing, which precludes its use in self-aligned MOS processing. This is not the case, with the refractory metals (i.e. Tungsten-W, Titanium-Ti, Molybdenum-Mo, and Tantalum-Ta). The applicability of these materials to VLSI interconnect applications has been considered (e.g. U.S. Pat. No. 4,629,635.) Extensive efforts have been directed towards developing chemical vapor deposition (CVD) of tungsten (W) thin films for low resistance interconnects in VLSI devices including self-aligned MOS devices.
By far the most widely employed precursor for depositing tungsten thin films by CVD is tungsten hexafluoride, (WF.sub.6). Processes for depositing tungsten films by CVD fall into two broad categories: the first is designated selective (N. E. Miller and I. Beinglas, Solid State Technology, December '82, p.85.) and the second is designated blanket deposition (K. C. Saraswate, et.al., IEEE Trans. on Elect. Dev., ED-30, No. 11, p. 1947, (1983)). The term "Selective" generally refers to the deposition of tungsten on silicon or metal substrates but not on silicon oxide or metal oxide substrates. The term blanket deposition generally refers to the deposition of tungsten on the entire surface of the substrate. Both types of depositions rely upon the reduction of WF.sub.6 by a reducing agent typically hydrogen, silane, or silicon or a combination thereof. Whether the process is selective or not is determined by the reaction conditions of the deposition such as the temperature, pressure and nature of the reagents used during the tungsten deposition.
The chemistry involved in depositing tungsten by CVD using WF.sub.6 has been extensively reviewed and discussed in several books including Refractory Metals for VLSI Applications, Editor R. S. Blewer, Material Research Society, 1986; ibid, Vol. II, Editor E. K. Broadbent, 1987; and ibid Vol. IV, Editors R. S. Blewer and C. M. McConica, 1989. The teachings of these books are incorporated in this disclosure by reference.
The invention of this disclosure relates to modifying the chemistry of the reduction of WF.sub.6 for depositing thin W film on a substrate. The prior art for the reduction of WF.sub.6 specifically on a silicon substrate to give tungsten films is summarized in equations 1-4 below: EQU 2 WF.sub.6 +3 Si.degree..fwdarw.2 W.degree.+3 SiF.sub.4 1) EQU WF.sub.6 +3 H.sub.2 .fwdarw.W.degree.+6HF 2) EQU 2 WF.sub.6 +3 SiH.sub.4 .fwdarw.2 W.degree.+3 SiF.sub.4 +6 H.sub.2 3) EQU WF.sub.6 +Si.sub.N H.sub.4N-2 .fwdarw.W.degree.+N(SiF.sub.4)+(2N-1)H.sub.2 4)
Where N=1,2, or 3
Equation 1 relates to the reduction of WF.sub.6 by a silicon substrate during the initial stage of W film deposition. The WF.sub.6 is reduced to the tungsten metal and the silicon is oxidized to the volatile SiF.sub.4. This can be a deleterious reaction which leads to erosion of the substrate, undercutting of masked areas, and deposition of porous films (e.g. E. K. Broadbent and C. L. Rumiller, J. Electrochemical Society, 131, No. 6, p. 1427, (1984) or the general references cited above). Other substrates such as aluminum metal also undergo a similar reaction with WF.sub.6 to deposit tungsten. These are self limiting reactions in that only very thin films can be deposited. Therefore, it is not desirable to deposit tungsten films on metallic or silicon substrates by the reduction of WF.sub.6 with the substrate when thicker films are required or substrate damage must be avoided.
Equation 2 represents the deposition of tungsten from WF.sub.6 by the reduction with hydrogen and is the most widely used technique for selective and blanket depositions. In the selective process, the H.sub.2 is activated for reduction of WF.sub.6 by chemisorption onto a tungsten surface. In combination with the chemistry summarized in equation 1, this results in film formation only on areas which were initially Si.degree., and no deposition on SiO.sub.2. The deposition temperature can range from 300.degree. to 500.degree. C. depending on the application. Selective deposition can also occur on other substrates which permit an initial tungsten metal deposition similar to that illustrated in equation 1.
Equations 3 and 4 represent a recent development in WF.sub.6 reduction for depositing tungsten films. There are now many references describing the use of silane or polysilane to deposit tungsten film from WF.sub.6. Several references are contained in the workshop proceedings on tungsten and other refractory metals for VLSI applications cited above. References from the literature include Park, H. L. et al Thin Solid Films 181, 35-93, 1989 and Rosler, R. S. et al J. Vac. Sci. Technol. 6(6) 1821-7, 1988 and from the patent art including U.S. Pat. Nos. 4,851,295 and 4,892,893. The teachings of these references are incorporated herein by reference. The reactions result in the deposition of W.degree. and generation of a volatile SiF.sub.4 from silane or polysilane thereby reducing silicon incorporation in the depositing film. Silane or polysilane reduction of WF.sub.6 is generally carried out in the presence of H.sub.2. Therefore, these deposition reactions are a combination of equations 1-3 or 1,2 and 4. Silane or polysilane additions aid in improving the deposition rate at low temperatures tending to enhance the selective of W.degree. deposition on Si.degree. verses SiO.sub.2.
There are disadvantages in using silanes for WF.sub.6 reduction and some of these disadvantages are: Silane and polysilanes are toxic (i.e. TWA of silane is 5 ppm) and pyrophoric materials and hence dangerous and expensive to handle. There have been several fatal explosions related to silane use in the electronics industry. In addition, the amount of silane used in the reactions as well as the deposition temperature must be carefully controlled to limit the amount of silicon incorporation into the films. Silicon incorporation in the film results in a higher electrical resistivity relative to the pure tungsten metal (Ohba, T. et al, Tungsten and Other Refractory Metals for ULSI Applications Vol. IV p. 17 and U.S. Pat. Nos. 4,851,295 and 4,892,893.) Apparently because of silicon incorporation resistivity values for silane deposited films in general are greater than those for hydrogen reduced WF.sub.6 films.
The mechanism of WF.sub.6 reduction by silane described by M. L. Yu, and B. N. Eldrige, in J. Vac. Sci. Technol. A7,625, 1989 comprises alternating deposition of tungsten and then silicon. The removal of silicon is accomplished by the reaction with WF.sub.6 to deposit tungsten according to equation 1. The deposition of tungsten and silicon followed by the removal of silicon results in the deposition of tungsten film with silicon impurity. Silane is known to deposit silicon under appropriate conditions but the feed material for depositing such films are generally thought to be restricted to silicon compounds with readily removable groups.
Herd S. R. et al in the Proceedings of the 1989 workshop on Tungsten and other Refractory Metals, Materials Research Society, Pittsburgh, Pa. 48-53 (1989) reported the reduction of WF.sub.6 using dichlorosilane H.sub.2 SiCl.sub.2 as a reductant. The authors concluded that the rate of tungsten deposition in this case was significantly lower relative to that of the simple H.sub.2 reduction carried out under similar conditions. The resistivities of the films were also higher than those obtained by hydrogen reduction i.e. 68 .mu..OMEGA.-cm vs 13 .mu..OMEGA.-cm. This would suggest that even a simple modification of the silane and polysilane reductant has a deleterious effect on the film quality.
S. Nishikawa et al J. Appl. Phys. p774-777, 67, 1990 describe the reduction of WF.sub.6 to tungsten films using mixtures of H.sub.2 and hexafluorodisilane, Si.sub.2 F.sub.6. The maximum rate enhancement was only twice that of the pure hydrogen reduction and the resistivity of the films was very sensitive to the ratio of Si.sub.2 F.sub.6 to WF.sub.6. Above a 0.5:1 molar mixture of Si.sub.2 F.sub.6 :WF.sub.6 there were large increases in film resistivity from their minimum values of approximately 10 .mu..OMEGA.-cm, presumably related to silicon incorporation into the film. Finally, Si.sub.2 F.sub.6 is a toxic, highly corrosive, and water reactive gas which requires special handling.
The presumptions in the literature relating to the mechanism for the reduction of WF.sub.6 by silane and polysilane along with examples of carbon incorporation in films deposited using organosilane precursors would lead to the conclusion that the groups bound to the silicon must be easily removed (i.e., the hydride ligand in silane), and that difficult to remove groups such as carbon containing organic moieties i.e. methyl, ethyl substituted silanes would result in the incorporation of carbon in the films. H. Du, et al in the Chemistry of Materialsp 569-571, 1, 1989 describe heavy carbon contamination of silicon nitride films prepared by CVD reactions of amino organohydrosilanes.
R. G. Gordon et al Chem. of Materials p 480-482, 2 1990 describe carbon incorporation in silicon nitride films from organoaminosilanes and similar findings of carbon incorporation have been associated with depositions of polysilicon and silicon carbide.
Although the methods described above and illustrated by equations 1 to 4 have been useful in depositing thin tungsten films on ceramic and metallic substrates, no one has yet disclosed a fully satisfactory method of depositing thin tungsten films at good deposition rates and at low temperatures without undercutting the masked areas, eroding the substrate, or using toxic or pyrophoric materials. As reported in some of the references previously cited films deposited from silane enhanced reduction show higher resistivities than 6.0.times.10.sup.-6 ohm-cm which is reported as typical for hydrogen reduced WF.sub.6 films. The deposition of thin tungsten films at low temperatures without eroding the substrate, undercutting the masked areas, and with using non-toxic and non-pyrophoric materials would be useful to the electronics industry in terms of increasing overall productivity and efficiency by reducing wastage.